1. Field of the Invention
The present invention relates to a formed polyimide article essentially consisting of a novel polyimide and particularly relates to a polyimide filament and a polyimide film.
2. Description of the Related Art
Aromatic polyimides have excellent mechanical properties and solvent resistance in addition to the highest heat resistance in organic polymers. For example, KAPTON and Vespel (Trademarks of E.I. Du Pont de Nemours & Co.) are polyimides obtained by reaction of bis(4-aminophenyl) ether with pyromellitic dianhydride and have been practically used in the form of film or molded articles. These polyimides, particularly polypyromellitimide prepared from aromatic diamine and pyromellitic dianhydride, have a straight chain polymer structure and high crystallinity. However, the research on polyimide filaments composed of polypyromellitimide has been found only in M.M. Katon, Polym. Sci. USSR. 21, 2756 (1980), although the material has possibility for use in filaments having great strength and high elasticity of modules. Poly(4,4'-biphenylenepyromellitimide) filament has relatively high strength in the filaments thus obtained, nevertheless the strength is only 6.9 g/d. The reason for such low strength is that melt spinning is impossible because of poor processability of conventional polypyromellitimide, dry spinning is also impossible because of insolubility in most solvents, and hence the filament can only be prepared by a wet spinning process. In the wet spinning process, unstable polyamic acid which is the precursor of polyimide is subjected to wet spinning, followed by hot water stretching and hot cyclization to obtain polyimide filaments.
Recently, Kanda et al. intended in Sengakushi, 40, T-480(1980) to enhance stretchability of polyimide by the sacrifice of high crystallinity of polypyromellitimide, and used tetracarboxylic acid dianhydride having a flexible linkage such as an ether bond or a carbonyl bond in the molecule. Thus, bis(3,4-dicarboxyphenyl) ether dianhydride or 3,3',4,4'-benzophenonetetracarboxylic dianhydride were reacted with 2-chlorobenzidine or 2-chloro-p-phenylenediamine as a diamine component to obtain polyimide having low crystallinity. The filament prepared from the polyimide obtained had a strength of 19.7 g/d.
As a result of introducing a crystallinity lowering structure in order to improve stretchability, modulus of elasticity is decreased to a low level of 1380 g/d and overall performance as a heat-resistance filament becomes unsatisfactory. Further, a highly crystalline polypyromellitimide which can expect a further increase in modulus of elasticity in view of its polymer structure was prepared from benzidine and pyromellitic dianhydride. The polyimide monofilament obtained from the polypyromellitimide had also inferior properties such as strength of 7.7 g/d and elastic modulus of 880 g/d. These properties also result from inferior processability of conventional polypyromellitimide. That is, the filament can only be prepared by wet spinning wherein a portion of polyamic acid precursor is chemically imidized and then spun and followed by stretching and heat treatment to give a polyimide filament.
In order to solve these problems, the present inventors have already found a novel polyimide which has both high crystallinity of polypyromellitimide and melt spinning ability and is represented by the formula (II): ##STR3## and have obtained a polyimide which has great strength and high modulus elasticity as disclosed in Japanese Patent Laid-Open 211319 (1988).
However, the polyimide represented by the above formula (II) has a high crystal-melting point (Tm) of 388.degree. C. and causes a problem that processing such as filament spinning must be carried out at a high temperature around 400.degree. C. Consequently, it has been desired to further improve processability without giving adverse effect on the crystallinity.
On the other hand, conventional polyimide prepared from bis(4-aminophenyl) ether and pyromellitic dianhydride as mentioned above cannot be melt-formed. Accordingly, a film is prepared by a so-called solution casting method wherein a polyamic acid precursor is cast, solvent is removed and the residual film is heat-treated. Consequently, problems of poor productivity and high cost have been encountered.
Much research has recently been carried out on thermoplastic polyimide in order to improve the processability of polyimide. One of the results is a noncrystalline polyetherimide having the polymer (a): ##STR4## The polyetherimide has a low Tg of 215.degree. C., and softens and can deform at a temperature above the Tg. On the other hand, the polyetherimide has disadvantages of no solder heat resistance and susceptibility to solvents. Japanese Patent Laid-Open 205124 (1987) has disclosed a novel crystalline polyimide having recurring structural units of the formula (II).
The polyimide can be melt-processed and also has great strength, high elastic modulus and excellent heat and solvent resistance. However, the polyimide is crystalline and must be melt-processed at temperature above Tg of the polyimide. High processing temperature of around 400.degree. C. is hence required for forming film by melt extrusion and fish eyes and die lines tend to occur. As a result, it is a problem that a high level of processing technique is required for producing good films. Further, the crystallization speed of the polyimide cannot be controlled and hence leads to a disadvantage that stretching conditions are limited to a very narrow range.
The present inventors have developed a process for controlling the rate of crystallization alone without impairing the high crystallinity of polypyromellitimide represented by the above formula (II) as disclosed in Japanese Patent Application 090674 (1989). Thereby they have found a novel polyimide having good processability and being capable of melt spinning and succeeded in obtaining a polyimide filament of high elastic modulus. Thus the present invention has been completed.